Conservation biological control using Australian native plants in a brassica crop system: seeking complementary ecosystem services

Habitat management can enhance the strength of ecosystem services but carries also the risk of leading to ecosystem disservices unless trophic links and ecosystem functions are well understood. In this study, inter cropping with Australian native flowering plants was compared with the use of an exotic plant, buckwheat (Fagopyrum esculentum) and naturally occurring weedy grass vegetation for the provision of multiple ecosystem services in a cabbage agroecosystem. Among the native plants studied, Mentha satureioides had the most comprehensive positive effects on pest management. Compared with the grassy control, M. satureiodides significantly enhanced parasitism of the pest, Plutella xylostella (L.) (Lepidoptera: Plutellidae) larvae and reduced pest Brevicoryne brassicae densities and was comparable to F. esculentum. Flowers of M. satureiodides appeared attractive to adult Pieris rapae and the adjacent cabbage foliage held high densities of this pest's eggs but, reflecting high densities of parasitoids and predators (especially spiders), larval densities were not elevated compared to other treatments. Lotus australis also enhanced natural enemies, especially coccinellids and spiders, but this did not translate into the ecosystem service of pest suppression. Swainsona galegifolia harboured elevated numbers of spiders but the adjacent crop did not benefit from enhanced enemy densities or suppressed pests. For complementary ecosystem services, pollinator abundance was enhanced by the exotic plant F. esculentum and two natives (L. australis and M. satureioides). Honey bees (Apis mellifera) were highly attracted to F. esculentum, while native bees were attracted to the native plants. Native butterflies were monitored as a proxy for benefit to native invertebrate biodiversity and two native plants (L. australis and M. satureioides) were found to be attractive. Soil biological activity, a proxy for soil-associated ecosystem services, was lower in the F. esculentum treatment compared to all other treatments. These results show scope for farmers to take advantage of particular, and potentially multiple, ecosystem services by incorporating native flowering plants into farming systems but highlights the importance of plant selection and the risk of trade-offs among services.